The present invention relates to a process for the distillation of readily polymerizable vinyl aromatic compounds. More particularly, the present invention relates to a process wherein styrene, substituted styrene, divinylbenzene and polyvinylbenzenes are subjected to elevated temperatures such as in the distillation thereof, wherein the amount of said materials polymerized during distillation is reduced over an extended period of time.
It is well known that vinyl aromatic compounds such as monomeric styrene, lower alkylated styrene, e.g., alpha-methyl styrene, and divinylbenzene polymerize readily, and furthermore, that the rate of polymerization increases with increasing temperature. Inasmuch as styrene and divinylbenzene produced by common industrial methods contain impurities, these compounds must be subjected to separation and purification processes in order to be suitable for most types of further industrial use. Such separation and purification is generally accomplished by distillation.
In order to prevent polymerization at the conditions necessary to distillation of vinyl aromatic compounds, various types of known polymerization inhibitors have been employed in connection with prior art distillation processes. For example, common inhibitors useful for inhibiting the polymerization of vinyl aromatics under distillation conditions include 4-tert-butylcatechol (TBC) and hydroquinone. It is preferred, however, to purify vinyl aromatics by using vacuum distillation techniques, whereby these commonly employed inhibitors are rendered unsuitable in view of the fact that they are effective only in the presence of oxygen. The partial pressure of oxygen in a vacuum distillation column is accordingly too low for these conventional inhibitors to be effective. Sulphur is perhaps the polymerization inhibitor most commonly employed to inhibit polymerization of vinyl aromatic compounds during distillation, since sulphur does provide effective inhibition in the absence of oxygen. While sulphur provides a reasonably effective inhibitor, its use in distillation processes results in one very significant disadvantage, namely, there is formed in the reboiler bottoms of the distillation column a valueless waste material which is highly contaminated with sulphur. This waste material furthermore represents a significant pollution or waste removal problem, although the disposal problem may be ameliorated somewhat by extracting the sulphur from the distillation bottoms and recycling it back into the distillation system, as described in U.S. Pat. No. 3,629,076. However, this method requires an expensive methanol extraction step.
Although many compounds are effective for inhibiting the polymerization of vinyl aromatic compounds under differing conditions, e.g., storage, other purification techniques, etc., for a number of reasons which are not entirely understood in view of the diverse and unpredictable results obtained, only extremely few of these compounds have proved to be of any utility for inhibiting vinyl aromatic polymerization under distillation conditions, particularly under vacuum distillation conditions. In addition, certain compounds which are useful for inhibiting polymerization of one type of vinyl aromatic compound, for example, styrene, have proved to be essentially ineffective for inhibiting polymerization of another species of vinyl aromatic compound, for example, divinylbenzene. A limited number of nitroso compounds have proven to be effective for inhibiting polymerization of styrene monomer during distillation. For example, N-Nitroso phenylhydroxylamine and p-nitroso-N,N-dimethylaniline are reasonably effective inhibitors for the distillation of styrene, although they are not particularly soluble in styrene monomer. On the other hand, N-Nitroso diphenylamine disclosed in U.S. Pat. No. 3,816,265, assigned to the assignee of the present application has been demonstrated to be a particularly effective polymerization inhibitor under vacuum distillation conditions for both styrene and divinylbenzene, whereas, N-Nitroso-methylaniline as disclosed in U.S. patent application Ser. No. 288,138, also assigned to the assignee of the present application, has been found to be an excellent polymerization inhibitor for styrene under vacuum distillation conditions. One of the most effective inhibitor systems known for divinylbenzene comprises a mixture of sulphur and N-Nitroso phenylhydroxylamine. In addition to the nitroid compounds, it has been found that m-nitro-p-cresol is an effective inhibitor. The use of such compound is described and claimed in copending U.S. application Ser. No. 749,406, filed Dec. 10, 1976.
In a typical distillation process for vinyl aromatic compounds utilizing a polymerization inhibitor, the mixture of vinyl aromatic to be distilled is generally contacted with the chemical polymerization inhibitor prior to being subjected to distillation conditions in the distillation apparatus. It remains as a significant problem today that the amount of polymer formed in the distillation apparatus and in the high purity product recovered therefrom is substantially higher than desired, and occasionally, that complete polymerization occurs inside of the distillation apparatus. For example, in the process of distilling crude divinylbenzene (a mixture containing divinylbenzenes, diethylbenzenes and monovinylbenzenes) to obtain high purity divinylbenzenes, even when inhibited with sulphur and TBC, a divinylbenzene product is obtained which contains significant quantities of polymer which are difficult to separate from the product and detrimental to the end use of such divinylbenzenes. Furthermore, the material which is removed from the bottom or reboiler area of the distillation apparatus is a highly polluting sulphur-containing waste material which must be disposed of.
It is therefore desirable to provide new polymerization inhibitors which are useful for styrene and vinylbenzenes under elevated temperatures such as those used under distillation conditions, particularly vacuum distillation conditions and which are not subject to the disadvantages outlined above.